[RFC 3927][1] is the specification for how it _should_ be implemented. It has a section [2.1. Link-Local Address Selection][2] which specifies the usage of a RNG and _recommends_ that the RNG should be seeded using the same initial value (e.g. a MAC address) every time.

```
   The pseudo-random number generation algorithm MUST be chosen so that
   different hosts do not generate the same sequence of numbers.  If the
   host has access to persistent information that is different for each
   host, such as its IEEE 802 MAC address, then the pseudo-random number
   generator SHOULD be seeded using a value derived from this
   information.  This means that even without using any other persistent
   storage, a host will usually select the same IPv4 Link-Local address
   each time it is booted, which can be convenient for debugging and
   other operational reasons.  Seeding the pseudo-random number
```

[1]: https://www.rfc-editor.org/rfc/rfc3927
[2]: https://www.rfc-editor.org/rfc/rfc3927#section-2.1